多种LVDS 的介绍

1、多种LVDS的介绍The Many Flavors of LVDSJohn Goldie - Manager of Interface ApplicationsThe LVDS standard for Low Voltage Differential Signaling is becoming the most popular differential data transmission standard in the industry. This is driven by two simple features of the bus, Gigabits milliwatts! It delivers the speed without consuming the power. In addition, it brings along a whole set of other benefits that include low voltage power supply compatibility, low noise generation, high noise rejection,

2、 robust transmission signals, and it allows for integration. For these reasons, it has been deployed across market segments wherever the need for speed exists. Even with all these benefits, there are some limitations in certain applications. These could be summed up with the following list of desires: support of multipoint bus configurations, operation from a lower yet power rail, and extended receiver common-mode range. This list of desires has spawned new variants of LVDS that in some cases pu

3、sh the performance even further into special applications.This article provides general information and key parameters for the original LVDS standard and the new flavors of LVDS that are in the market place today. Before describing the different flavors of LVDS, it is useful for the reader to have a basic understanding of different bus configurations.The three common types of bus configurations are: Point-to-Point Multidrop MultipointPoint-to-Point is the simplest bus configuration.The source is

4、 at one end, then the interconnecting media, and at the other end is the termination and receiver.Due to the clean signaling path, a point-to-point bus supports the highest data rates.Gigabit links are easily possible with this bus structure.This bus is also characterized by the single direction flow of information (simplex).If information needs to flow in the other direction, then a separate channel typically carries that information.With this approach, full duplex transmission can occur (infor

5、mation flows in both directions at the same time).Crosspoints, repeaters and distribution buffers can be used to support many different applications with high-speed point-to-point links to expand their versatility.Multidrop, as the name implies, drops the information off at several locations along the bus.This is an efficient bus in interconnect terms when multiple locations need the same information.The driver (source) is located at one end of the bus, and once again, termination is at the far

6、end as in the case of the point-to-point bus. There are two or more receivers located along the bus.These are connected to the main line with short interconnects called stubs.Stub lengths need to be minimal length to avoid adverse transmission line effects.This configuration has a max data rate from 400 to 800 Mbps depending upon the stub effect and number of loads.Multipoint is the most flexible bus because the information source can now be from any node on the bus (but from only one at a time)

7、 to all other locations on the bus. Since information can flow in either direction, or both directions when driven from the middle of the bus, the bus requires termination at both ends. The benefit of this bus is the interconnect density reduction because all nodes share a common bus. The negative side of this approach is that a control scheme or protocol overhead is required to control the bus.Also if all nodes had important information to send at the same time, the problem becomes evident that

8、 only one can send at a time.Some information will be late, as that driver would need to wait till it gains control of the bus. For this reason, multipoint buses tend to be used in master/slave applications and also test and maintenance buses.Today there are many flavors of LVDS on the market from a variety of vendors.Some devices are generic interface parts while others are much more application specific solutions.The most common and original is LVDS which is defined in the TIA/EIA-644 standard

9、.There are also some others standards and vendor specific implementations that are similar to LVDS.The next section describes these flavors.LVDSNational Semiconductor first introduced Low Voltage Differential Signaling as a standard interface device back in 1994.National realized that the demand for bandwidth was increasing at an exponential rate while users also desired reductions on power dissipation.This exceeded the capability of RS-422 and RS-485 differential transmission standards in speed.ECL (or PECL) was available at the time, but incompatibility with standard logic levels, negative power rails and high chip power dissipation limited its wide spread acceptance.Thus, National invented of LVDS. LVDS is differential, using two signal lines to convey information.While sounding like a penalty, this is actually a benefit.The cost is two traces (or conductors) to convey t

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